PROJECT SUMMARY Hemophilia A (factor VIII deficiency) and von Willebrand disease (VWF deficiency) are well recognized severe and/or common bleeding disorders that require frequent treatment with IV infusion of clotting factors ? FVIII and VWF respectively. Studies by our laboratory and others suggest a unique relationship between VWF and FVIII that will be explored further in this proposal. While prophylaxis to prevent bleeding is the current standard of care, this requires infusions several times a week. While gene therapy could be curative, studies in patients have yet to achieve adequate efficacy except in patients with hemophilia B (factor IX deficiency). Over the past several years our laboratory has focused on delivering FVIII by targeting expression to the platelet so that FVIII is stored together with VWF and this approach results in therapeutic efficacy even in the presence of high titer FVIII inhibitors. The local release of the FVIII/VWF complex circumvents the immediate inactivation of FVIII and permits cessation of bleeding even in the face of high-titer FVIII inhibitory antibodies. In Aim-1 a second animal model has been developed to study the role of the VWF and FVIII complex in gene therapy of hemophilia A even in the context of FVIII inhibitors in a rat model that has both spontaneous bleeding, a high propensity to form FVIII inhibitory antibodies, and has a blood volume size that enables easy sequential sampling unlike the mouse where sequential time points require animal sacrifice rather than sequential phlebotomy. This can substantiate the efficacy of 2bF8 in an inhibitor prone model and potentially demonstrate efficacy in preventing spontaneous bleeding ? a characteristic that cannot be studied in the mouse or the dog were spontaneous bleeding is difficult to study. While it is well recognized that VWF prolongs the survival of FVIII, in Aim 2 we will explore the functional importance of this VWF delivering FVIII to the site of the evolving thrombus. In Aim 3 we will further study VWF and FVIII synthesis and the cells that produce them as a complex and as individual proteins. These three aims will further our understanding of the biologic role for the complex of FVIII and VWF in normal hemostasis and thrombosis and through these interactions offer a strategy for effectively treating hemophilia A subjects with FVIII inhibitors through a unique gene therapy strategy that ectopically expresses FVIII with VWF in platelets.